Supported chromium catalysts have long been used in olefin polymerisation.
In such products key parameters are pore volume, surface area and pore diameter. Whilst surface area and pore volume can to some extent be varied independently of one other, the pore diameter of the resulting support or catalyst is directly related to the surface area and pore volume. In particular, assuming cylindrical pores the average pore size (APD) in Angstroms is calculated as 40,000 PV(cm.sup.3 /g)/SA(m.sup.2 /g). Thus all three parameters influence catalyst performance and particularly the structure of the polyethylene produced. In general terms higher surface areas, pore volumes and pore diameters result in increased activity, whereas the molecular weight of the resultant polymer decreases with increase in average pore diameter. Clearly, because APD is related to surface area and pore volume as indicated above, changes in molecular weight are also effected by changes in surface area and pore volume.
U.S. Pat. No. 4,981,831 discloses a process for manufacturing a catalyst wherein a support is first formed by forming a silica titania cogel which is then first aged at neutral pH and then aged an alkaline pH. The twice aged cogel is then spray dried to form a xerogel and is then impregnated with chromium. This process requires two aging steps which altogether are typically between 13 and 23 hours long. The obtained catalyst has a low surface area (300 to 400 m.sup.2 /g), a pore volume of between 0.7 and 1.1 cc/g and a mean pore diameter of between 60 and 120 Angstroms.
There is therefore a need for a process which avoids a two step aging while producing a catalyst with the required mean pore diameter and a satisfactory activity.
It has now been discovered that it is possible to avoid a two-step aging process as well as a spray drying step while nevertheless obtaining a product, with an increased surface area and keeping a pore volume above 0.9 cc/g and maintaining an average pore size diameter of about 70 to 100 Angstroms, which exhibits satisfactory activity.